The invention relates to a method of measuring the amount of silicone coating on a moving paper or cardboard web, wherein the amount of the coating is measured substantially continuously.
The invention further relates to an apparatus for measuring the amount of silicone coating on a moving paper or cardboard web, the apparatus comprising a radiation source for producing a beam, means for directing the beam at a material to be measured, a detector for measuring the beam arriving from the material to be measured, and means for processing a signal from the detector, the apparatus being arranged to measure the amount of the coating, substantially continuously.
When silicone-coated products are being manufactured, it is important to monitor the amount of the silicone coating on a moving substrate, such as a paper or cardboard web. Silicone coating is used for paper grades that require small release strength. An example is release paper for adhesive paper. The amount of the silicone coating in a particular paper grade should be kept constant. A typical amount of silicone varies from 0.5 to 1.5 g/m2. Silicone is one of the most expensive materials in the manufacture of paper, and therefore measuring the amount of silicone accurately reduces costs considerably and provides products that are more uniform in quality.
The amount of a coating on a moving web can be measured, for example, with a method based on the difference between dry weights. In this method the basis weight of the web is measured by means of beta radiation absorption, and moisture is measured by infrared measurement. Both measurements are carried out before and after a coating station. The wet weight is subtracted from the basis weight to obtain the dry weight, and the difference between the dry weights determines the amount of the coating. The equipment is rather complicated and the accuracy of the final result depends on the accuracies of four meters. In practice, the accuracy of the measurement based on the difference between dry weights varies approximately from 0.5 to 1 g/m2. Therefore, the arrangement is not at all suitable for measuring the amount of silicone coating, where the accuracy should be about 0.1 g/m2. Also, the arrangement is expensive since it comprises four meters and two measuring frames.
The amount of the silicone coating is also measured by an X-ray fluorescence method, where the target of measurement is subjected to X-radiation radiation, fluorescence peaks caused by silicon atoms are measured, and the amount of the silicone coating is determined on the basis thereof. However, the fluorescence includes both the effect of the silicon atoms in the silicone coating and the effect of, for example, silicon atoms contained in clay used as a filler in the base paper. Therefore the measurement result is not accurate. U.S. Pat. No. 4,639,942 discloses an X-ray fluorescence method where the accuracy is improved by the geometry of the equipment and by calculation. However, the arrangement is highly complicated and rather slow due to the required calculation. Further, the use of X-radiation in the measuring apparatus is hazardous to those working near the apparatus.
The prior art also teaches measurement of the amount of the silicone coating in the near-infrared range, where a weak absorption peak can be located in the coating on a wavelength of about 2.3 micrometers. However, due to the weakness of the absorption it is difficult to achieve sufficient accuracy of measurement. Another problem is that binders, such as latex, used in the paper and the coating absorb in this range, which makes the measurement inaccurate if the effect of the interfering component is not measured before the siliconization.
EP 0,268,029 discloses a continuous method of measuring the amount of silicone coating on a moving paper web. The method comprises measuring the gloss of the web surface before and after the coating process. The difference between the measured gloss values is then used to determine the amount of the silicone coating. The arrangement requires two devices for measuring the gloss, which makes the measuring equipment cumbersome, complicated and expensive. Further, the results of gloss measurements are rather unreliable and not very accurate. Therefore, the amount of the silicone coating calculated on the basis of the gloss measurements is often erroneous and misleading.
The purpose of the present invention is to provide a method which avoids the aforementioned drawbacks.
The method according to the invention is characterized by directing an infrared beam at the moving paper or cardboard web and measuring an absorption peak that is characteristic of the silicone coating in the middle infrared range on a wavelength of from 2.5 to 5 micrometers.
Further, the apparatus according to the invention is characterized in that the radiation source produces an infrared beam, and that the apparatus is arranged to measure the amount of the silicone coating by measuring an absorption peak that is characteristic of the coating in the middle infrared range on a wavelength of from 2.5 to 5 micrometers.
The invention is based on the idea of directing an infrared beam at a moving paper or cardboard web, measuring an absorption peak that is characteristic of the silicone coating in the middle infrared range on a wavelength of 2.5 to 5 micrometers, and determining the amount of the silicone coating on the basis of the measurement. The idea of a preferred embodiment is to measure an absorption peak that is characteristic of a methyl group in the silicone coating in a wavelength range of about 3.4 micrometers.
An advantage of the invention is that the amount of the silicone coating can be measured very accurately and reliably. Measurement can be carried out by one meter and, in the case of reflection measurement, by half a measuring frame.